The invention relates to a method and apparatus for gas chromatography analysis of samples.
To use gas chromatography to investigate small quantities of components present in gases or liquids, such as foreign substances or pollutants or impurities, it is known firstly to enrich these in order then to feed them into a gas chromatograph via an appropriate feeding system. However, problems occur in this case when the collected samples contain moisture such as is the case, for example, when pollutants contained in the air are enriched, since the moisture contained in the air is then also enriched.
However, water severely disturbs a gas chromatography system, and likewise the analysis, in the case of which, for example, a substantial loss in sensitivity occurs in the mass spectrometer. The presence of water in separation columns alters the retention time, doing so, specifically, as a function of quantity and differently for different substances, thus creating the need to eliminate this as completely as possible in order to obtain reliable measurement results.
It is known to eliminate the moisture which is present in samples to be chromatographically analyzed by osmosis. However, this has the disadvantage that polar components are also eliminated in the process, while non-polar components remain essentially uninfluenced. However, the elimination of polar components other than water falsifies the chromatogram.
Also known are packed capillary columns which exhibit a temperature-dependent adsorptivity with reference to water, so that given appropriate setting, low-boiling components are passed while higher-boiling components and water are retained.
It is an object of the invention to provide a method for gas chromatography analysis of samples which permits reliable gas chromatograms to be obtained from samples containing water.
It is a further object of the invention to provide an apparatus for gas chromatography analysis of samples which permits reliable gas chromatograms to be obtained from samples containing water.
According to the invention a method for gas chromatography analysis of a sample after preceding thermodesorption, in which the components to be separated and water are contained, is provided,
wherein the thermodesorbed sample is transferred by means of carrier gas into a first polar separation column which retains higher-boiling components and water and passes low-boiling components,
said low boiling components being led, past a branching device which leads, on the one hand, to a second polar or non-polar separation column and, on the other hand, to a non-polar separation column, to the non-polar separation column in a fashion excluding access to the second polar or non-polar separation column,
after which the higher-boiling components and the water are lead to the second polar or non-polar separation column in a manner excluding access to the non-polar separation column,
the water being eliminated upstream of the second polar or non-polar separation column by means of cryofocussing.
According to the invention, further an apparatus for gas chromatography analysis of a sample is provided, comprising:
a thermodesorption device for holding a sampling tube;
a first polar separation column being connected downstream of the thermodesorption device;
a branching device being connected downstream of the first polar separation column;
a non-polar separation column;
a second separation column being of the group of a polar and a non-polar separation column;
wherein said branching device being switchable over between said non-polar separation column; and
a device for eliminating water which is connected upstream of the second separation column.
By virtue of the fact that according to the present invention use is made as a precolumn of a polar separation column with a stationary phase, which water does not initially have the effect of separating it preliminarily into two fractions, higher-boiling components and water can be retained at the beginning, while low-boiling components are passed. The low-boiling components are separated on the non-polar separation column via a pneumatically closeable bifurcation which leads, on the one hand, to a non-polar separation column for gases and, on the other hand, via a cryofocussing device, to a further polar or non-polar separation column, whereupon after pneumatically switching over the bifurcation the water with higher-boiling components is eliminated in the region of the cryofocussing device, whereupon the higher-boiling components are separated in the polar or non-polar separation column downstream of the cryofocussing device. In addition, in this case the water elimination with subsequent separation and analysis of a sample, and the separation on the further separation column with subsequent analysis of another sample, can be carried out simultaneously.
In this case, not only gaseous but also liquid samples which contain water can be taken automatically by means of the apparatus.
Further objects, embodiments and advantages of the invention will become apparent from the following description and the claims.